The output power of mobile terminals, such as mobile phones, Personal Digital Assistants (PDAs), and the like, is controlled by a power amplifier in the transmit chain. For open loop power amplifiers, the output power of the power amplifier, and thus the mobile terminal, is a function of the load impedance seen at the output of the power amplifier. Ideally, the load impedance is a constant load. However, in reality, there are variations in the load impedance due to variations in the impedance of the components in the transmit chain following the power amplifier, such as a duplexer, with frequency and temperature, and variations in the impedance of the antenna of the mobile terminal caused by proximity of the antenna to foreign metallic objects and the user's body. For open loop power amplifiers, the variations in the load impedance result in variations in the output power of the power amplifier. Such output power variations decrease the quality of service (QoS), increase the probability of dropped calls in fringe areas, increase the liability of mobile terminal manufacturers in terms of tolerated output power versus Specific Absorption Ratio (SAR), and increase the peak current of the power amplifier.
In order to reduce output power variations due to variations in the load impedance, it is desirable to detect the output power of the power amplifier. Traditional approaches for detecting the output power use a directional coupler at the output of the power amplifier to provide a signal representative of the output of the power amplifier to a power detector. The power detector then processes the signal to detect the output power of the power amplifier. However, the directional coupler is an additional component that adds loss to the transmit path, which reduces the efficiency of the mobile terminal, and increases the cost of manufacturing the mobile terminal.
Thus, there remains a need for a system for detecting the output power of a power amplifier that is tolerant to variations in load impedance and eliminates the need for a directional coupler.